Recently, there has come to be extensively used an endoscope whereby an elongate insertable part can be inserted into a body cavity to observe or diagnose an internal organ or the like within the body cavity without incising it or to cure it by using a treatment tool.
In case the insertable part of the above mentioned endoscope is inserted into a body cavity to observe an affected part or the like, if the affected part is in the forward direction of the axis of the tip side of the insertable part, it will be easy to observe the affected part with a straight sight type endoscope. On the other hand, in case an inner wall surface of a body cavity on the side intersecting at right angles with the inserting direction is to be observed, it will be easy to observe such inner wall surface with a side sight type endoscope. It is desirable to use endoscopes adapted to these positions to be observed. However, the endoscope is so expensive that, in the prior art example disclosed, for example, in Japanese Patent Laid Open Nos. 70744/1981 and 85324/1981, an adaptor provided with an optical system different in the visual field direction or the like is fitted to the tip of the insertable part of an endoscope to be used so that the observation may be made in a different visual field direction or different angle of vision.
In the above mentioned prior art example, such illuminating light transmitting means as a prism or light guide is used. For example, in case an adaptor is fitted to the tip of a straight sight type endoscope so that a side sight direction may be observed, a light guide arranged within to transmit an illuminating light will be higher in the illuminating light transmitting efficiency than a prism arranged within and therefore it will be desirable to use a light guide. In case a light guide is used, it will have to be bent. In the above mentioned prior art example, the entrance end surface of the light guide on the adaptor side is connected to the exit end surface of the light guide on the endoscope side so that the axial directions of both light guides in their connecting part may coincide with each other and, on the forward side of this part, the light guide within the adaptor is arranged as bent substantially by 90 degrees.
It is disclosed in U.S. Pat. No. 4,195,904 that an adaptor having an optical system which can vary the illuminating and observing directions is provided in the tip part of an endoscope.
However, there has been a defect that, in the connection and arrangement of the light guide in the above mentioned prior art example, the radius of curvature of the light guide is so small that the light guide is likely to be broken. Also, there has been a defect that, in fitting a straight sight type endoscope with a straight sight type adaptor for varying the angle of vision, the positions of arranging the observing optical system and illuminating optical system within the adaptor are likely to interfere with each other, only small optical systems can be arranged and therefore it is difficult to secure a sufficient resolving power.